In recent years, as an information recording medium, optical disks such as CDs (Compact Disks) and DVDs (Digital Versatile Disks) have become predominant. A reproducing device of the optical disks reproduces recorded data based on a change in intensity of reflected light of laser light irradiated along a track of the optical disk.
FIG. 1 shows a schematic diagram of an optical pickup device that irradiates laser light and detects reflected light. As a laser light source, a semiconductor laser element 2 that is small in size and low in power consumption is used. Laser light exited from the semiconductor laser element 2 is focused on a surface of an optical disk 8 by a collimating lens 4 and an objective lens 6. Focused laser light is irradiated along a track of the optical disk 8 and the optical disk 8 returns reflected light whose intensity changes in accordance with data recorded along the track of the optical disk 8 to the optical pickup device.
A polarization beam splitter 10 disposed on an optical path transmits only a parallel polarization component (p wave) of incident light. Reflected light from the optical disk 8, after transmitting a quarter wave plate 12, together with a rotation by the quarter wave plate 12 at the time of irradiation, rotates by 90° in a polarization plane and enters the polarization beam splitter 10 as a perpendicular polarization component (s wave). The polarization beam splitter 10 reflects reflected light having a rotated polarization plane in a direction different from that of irradiation light. The reflected light separated from the irradiation light by the polarization beam splitter 10 enters a photodetector 18 through a focusing lens 14 and a cylindrical lens 16.
An optical disk reproducing device, while detecting data based on the reflected light, servo-controls a positional relationship between the optical pickup device and the optical disk 8. Specifically, tracking servo control for irradiating the laser light along a centerline of the track and focus servo control for keeping a distance between the optical disk 8 and the optical pickup device constant are carried out. In order to obtain information for such servo control, as the photodetector 18, a semiconductor device receiving a reflected light image divided into a plurality of partitions is used. Furthermore, the cylindrical lens 16 is disposed to carry out the focus servo control.
Incidentally, in the focus servo control, based on an output signal of the photodetector 18, an actuator variably controls a position of the optical pickup device to keep the distance from the optical disk 8 constant. Thereby, an amount of reflected light corresponding to a displacement of a focus of irradiation light on a surface of the optical disk 8 can be suppressed from fluctuating and noise superposed on the light receiving signal corresponding to data can be suppressed.
FIGS. 2 through 4 are schematic diagrams showing a light receiving portion of the photodetector 18 and a reflection light image on the light receiving portion. According to a principle of an astigmatism method, an image of reflected light past the cylindrical lens 16, in accordance with a distance d between the optical disk 8 and the objective lens 6, varies in a dimensional ratio in two perpendicular directions (in FIG. 1, a vertical direction in a page and a normal direction to the page). Specifically, when a distance d is a target value, as shown in FIG. 3, an image of reflected light is set so as to be a perfect circle 30. For instance, when the distance d is excessive as shown in FIG. 2, an image of reflected light becomes a vertically long ellipse 32 and, on the other hand, when the distance d is insufficient as shown in FIG. 4, an image of reflected light becomes a horizontally long ellipse 34.
The photodetector 18 has a light receiving portion that is divided into 2×2=4 partitions 36 and each of the partitions constitutes a light receiving element that outputs a light receiving signal. The photodetector 18 is arranged so that diagonal directions of a 2×2 square arrangement of the light receiving elements, respectively, may coincide with axes of the vertically long ellipse 32 and the horizontally long ellipse 34. When the photodetector is thus arranged, in FIGS. 2 through 4, based on a difference between a sum of output signals of two light receiving elements arranged on a diagonal line in a vertical direction and a sum of output signals of two light receiving elements arranged on a diagonal line in a horizontal direction, shapes of the respective reflection images can be distinguished and thereby the shape can be used to control distance d.
Since a data rate read from an optical disk is very high, the photodetector 18 is constituted of a semiconductor device that uses a PIN photodiode high in response speed. FIG. 5 is a schematic sectional view of an existing photodetector 18. The drawing expresses a sectional view that goes through two adjacent light receiving elements and is vertical to a semiconductor substrate. The semiconductor device has a p+ region that becomes an anode region 42, which is formed on a surface of a P-type semiconductor substrate 40. Above the anode region 42, an i layer 44 that is low in impurity concentration and high in resistivity is formed by an epitaxial growth method. In the i layer 44, an isolation region 46 that is made of a p+ region and continues to the anode region 42 is formed at a position corresponding to a boundary of the light receiving element. Furthermore, an n+ region that becomes a cathode region 48 is formed on a surface of the i layer 44.
The anode region 42, the i layer 44 and the cathode region 48 constitute a PIN photodiode that becomes a light receiving element of the photodetector 18. The anode region 42 and the cathode region 48, respectively, are connected to voltage terminals and a reverse bias voltage is applied therebetween. In a reverse bias state, in the i layer 44, a depletion layer is formed and electrons generated in the depletion layer by absorption of incident light move to the cathode region 48 due to an electric field in the depletion layer, followed by being output as a receiving light signal. Here, the isolation region 46, as mentioned above, reaches the anode region 42 from a surface of the i layer 44. Thereby, the i layer 44 is divided for every light receiving element and the crosstalk between light receiving elements can be inhibited.
A thickness of the i layer 44 is set equal to or more than a substantial absorption length of detected light in a semiconductor. For instance, an absorption length of silicon to light of a 780 nm or 650 nm band that is used in a CD or DVD is substantially 10 to 20 μm. The p+ layer of the isolation region 46 is formed by pressing, after the ion implantation, in a depth direction by means of thermal diffusion. At that time, in the thermal diffusion, the p+ region is expanded not only in a depth direction but also in a horizontal direction. In this connection, when the i layer 44 is relatively thick, in order to form an isolation region 46 restricted in a width, the i layer 44 is formed divided into a plurality of times of epitaxial growth. In this case, every time when an epitaxial layer 48 is formed, the ion injection and thermal diffusion are carried out from a surface thereof and as a result an isolation region 52 reaching a bottom surface of the epitaxial layer 48 is formed. When the epitaxial layers 48 and isolation layers 52 are thus layered, the isolation region 46 extending in a depth direction can be formed while suppressing a width.
Thus, when an i layer relatively thick such as 10 to 20 μm is formed, the formation of an epitaxial layer 50 and an isolation layer 52 is repeated a plurality of times. Accordingly, there is a problem in that a semiconductor device that constitutes an existing photodetector 18 becomes high in manufacturing cost. There is another problem in that, for a part of a junction area of the isolation region 46 and the i layer 44, a capacitance between terminals of an anode and a cathode increases and as a result the high-speed responsiveness that is a feature of the PIN photodiode is impaired.
Patent literature 1: JP-A-10-107243
Patent literature 2: JP-A-2001-60713